1. Field of the Invention
The present invention relates to an air/fuel ratio control technology for purifying exhaust gas emitted from an internal combustion engine, a combustor or the like, and more particularly to an exhaust gas purifying system capable of efficiently purifying nitrogen oxides in the exhaust gas by effective use of hydrogen.
2. Description of the Related Art
In order to improve fuel consumption of the internal combustion engine, an air/fuel ratio of a mixture supplied to the internal combustion engine is controlled to an air/fuel ratio of a lean side, in which A/F is about 22 (A: air mass, F: fuel mass), that is a fuel ratio is smaller than a stoichiometric air/fuel ratio of A/F=14.7. However, a three-way catalyst used for purifying NOx in the exhaust gas has highest purification efficiency in the stoichiometric air/fuel ratio. Accordingly, since the air/fuel ratio control to the lean side reduces the NOx purification efficiency of the three-way catalyst, a great volume of nitrogen oxides, i.e., NOx, is discharged into the atmosphere.
Therefore, for the internal combustion engine capable of executing lean-burning, purifying the nitrogen oxides by an NO trap-catalyst containing an NOx adsorbent for trapping the nitrogen oxides is employed. This NOx adsorbent traps NOx at the time of a lean air/fuel ratio, and reduces the trapped NOx by a reducer (HC, CO, or H) at the time of a rich air/fuel ratio to discharge and purify the NOx. The reducers not used for the discharging and purifying of the NOx are removed by oxidation.
However, in the case of the NOx adsorbent contained in the NOx trap-catalyst, because of a limitation on the volume of nitrogen oxides to be trapped, lean-burning cannot be continued for a long time. Thus, in order to discharge and purify the trapped nitrogen oxides, control must be executed to temporarily enrich an air/fuel ratio. Incidentally, control conditions for enrichment, i.e., control conditions of a value of an A/F ratio and a maintenance time of a rich state, vary depending on a gas flow rate or a gas flow velocity. The gas flow rate is represented by a space velocity (SV) or exhaust gas passed through the catalyst. The space velocity is a value obtained by dividing a gas flow rate (1/min) by a catalyst volume (1). Hereinafter, in the specification, the gas flow rate means a gas space velocity (SV) unless specified otherwise.
As an increase of a gas flow rate brings about an increase of a load on the internal combustion engine, an exhaust gas temperature is raised to increase a temperature of the NOx trap-catalyst. Following this temperature increase, the amount of NOx discharged from the NOx trap-catalyst is also increased. On the other hand, the increased gas flow rate reduces a contact time between the exhaust gas and the catalyst to lower reaction efficiency and, consequently, NOx purification efficiency is also reduced. Thus, control is executed to reduce an air/fuel ratio (A/F)r more at the time of enrichment or to prolong the time of enrichment (tr) as the gas flow rate or the gas flow velocity is increased.
The above NOx purifying method is very effective in a relatively high temperature region of 300° C. or higher. However, the NOx purification efficiency is considerably reduced in a temperature region lower than this region. This is attributed to the discharging difficulty of the trapped nitrogen oxides from the NOx adsorbent in the low temperature region. Investigation by the inventors et al. reveals that carbon monoxide (CO) and hydrocarbon (HC), especially CO, suppresses the discharging of the nitrogen oxides. However, it has been found that use of only hydrogen as a reducer greatly promotes the discharging of the nitrogen oxides to dramatically improve NOx purification performance. Based on such a knowledge, Japanese Patent Application Laid-Open No. 2001-234737 discloses a system combining a hydrogen-catalyst which is a hydrogen enriching means, and an NOx purification catalyst, in which among reducers, i.e., CO, HC and hydrogen (H2), generated in enrichment, the CO and the HC are selectively reduced, and the hydrogen is increased to be supplied.